In duplex radio communication, to make a transmitter and a receiver in a frequency division multiplexing (FDM) system share an antenna, a duplexer needs to distinguish a transmitted signal from a received signal. Generally, the duplexer is composed of a transmitter filter, a receiver filter, a combinational circuit, and so on. The combinational circuit includes reactive components such as transmission line, resistor, and capacitor. The combinational circuit is essentially a phasing circuit that can reduce mutual impact between the transmitter filter and the receiver filter.
Generally, duplexers include cavity duplexer, dielectric duplexer, surface acoustic wave (SAW) filter, bulk acoustic resonator (BAR), and film bulk acoustic resonator (FBAR). The FBAR is a duplexer based on a bulk acoustic resonance technology, and converts the electric energy into acoustic waves to generate resonance through a converse piezoelectric effect of a piezoelectric film. The wave velocity of the acoustic wave is less than the wave velocity of the electromagnetic wave by about 5 orders of magnitude. Therefore, under the same working frequency, the size of the FBAR may be far less than the size of the electromagnetic-based dielectric porcelain device. Meanwhile, because the bulk acoustic wave has a greater power bearing capability than the surface acoustic wave, the FBAR has a better power capacity feature than the surface acoustic filter.
With development of communication technologies, the operator requires smaller and smaller size of the communication system. Especially, the emergence of new base station forms imposes more intense requirements on the small size of the system. Currently, many discrete devices in the radio part have been replaced. Especially, the discrete devices in the receiver, such as low noise amplifier (LNA), variable gain amplifier, and intermediate frequency filter, have been integrated into a radio integrated circuit. It is very significant to replace cavity duplexers with small-sized duplexers. In a multi-carrier transceiver, a high requirement is imposed on the duplexer suppression (namely, the ratio of signals leaked to the receiving channel to signals transmitted by the transmitter; smaller value of this ratio means weaker interference to the received signals). Traditional small filters can hardly fulfill the strict requirement imposed by the base station. Therefore, how to reduce the suppression requirement is a linchpin of reducing the size of the duplexer.
A solution to reducing the suppression requirement imposed by the transmitter onto the receiver in the prior art is to simulate interference cancellation. In this solution, an amplitude phase regulator adjusts the phase of the transmitted signal that is coupled from the transmitting channel in a interference cancellation channel, and outputs the signal; on the receiver side, the transmitted signal whose phase is adjusted by the amplitude phase regulator is overlaid with the transmitted signal leaked from the transmitting filter on the transmitting channel, thereby cancelling the interference caused by the transmitted signal onto the received signal.
The inventors of the present invention through their research find that in the prior art, due to the outband (band outside the transmitting band) filter feature of the transmitting filter on the transmitting channel, the outband amplitude and phase response fluctuates sharply, the delay changes sharply, but the receiving band of the receiving filter in the interference cancellation channel is a passband (a passband is a band outside the transmitting band of the transmitting filter), and the amplitude and phase response of the receiving filter is relatively constant compared with the outband amplitude and phase response of the transmitting filter on the transmitting channel. Therefore, even if the amplitude phase regulator can adjust the phase of the transmitted signal output by the transmitting filter on the interference cancellation channel, the phase of the transmitted signal is not necessarily opposite to the phase of the transmitted signal output by the transmitting filter on the transmitting channel after the phase is adjusted. Consequently, the interference caused by the transmitted signal onto the received signal is not cancelled, and even may be exacerbated. For example, due to the outband phase fluctuation of the transmitting filter on the transmitting channel, after the phase is adjusted by the amplitude phase regulator, the phase of the transmitted signal coupled from the transmitting channel is just the same as the phase of the transmitted signal output by the transmitting filter on the transmitting channel. In this case, the interference caused by the transmitted signal onto the received signal is not cancelled, and even increased drastically.